Search Results for Laps

During autofrettage of a thick-wall steel pressure vessel, a crack developed through the wall of the component. Certain forged pressure vessels are subjected to autofrettage during their manufacture to induce residual compressive stresses at locations where fatigue cracks may initiate. The results of the autofrettage process, which creates a state of plastic strain in the material, is an increase in the fatigue life of the component. Analysis (visual inspection, 50x/500x unetched micrographs, and electron microprobe analysis) supports the conclusion that the fracture toughness of the steel was exceeded, and failure through the wall occurred because of the following reason: the high level of iron oxide found is highly abnormal in vacuum-degassed steels. Included matter of this nature (exogenous) most likely resulted from scale worked into the surface during forging. Therefore, it is understandable that failure occurred during autofrettage when the section containing these defects was subjected to plastic strains. Because the inclusions were sizable, hard, and extremely irregular, this region would effect substantial stress concentration. No recommendations were made.

Alloy steel forgings used as structural members of a ski chair lift grip mechanism were identified to have contained forging laps (i.e., sharp-notched discontinuities) during an annual magnetic particle inspection of all chair lift grip structural members at a mountain resort. The material was confirmed to be 34Cr-Ni-Mo6. A heavy oxide on the dark area of one of the broken-open laps was revealed by scanning electron microscopy in conjunction with EDS. A bright area that contained ductile dimple rupture was observed adjacent to the dark area. The oxidized portion of the fracture was established to be the preexisting forging lap while the bright area was created during the breaking-open process. As a corrective action all forgings showing laps were recommended to be removed from service. Critical review and revision of the forging process and revisions to the nondestructive evaluation procedures at the forging supplier was recommended.

A connecting rod (forged from 15B41 steel and heat treated to a hardness of 29 to 35 HRC) from a truck engine failed after 73,000 Km (45,300 mi) of service. A piece of the I-beam sidewall of the rod, about 6.4 cm (2 in.) long, was missing when the connecting rod arrived at a laboratory for testing. Analysis (visual inspection, 100x nital-etched micrograph, fluorescent magnetic-particle testing, and metallographic examination) supported the conclusion that the rod failed in fatigue with the origin along the lap and located approximately 4.7 mm below the forged surface. The presence of oxides may have been a partial cause for the defect. Recommendations included better inspection of the forgings by fluorescent magnetic-particle testing before machining.